1. Field of the Invention
The present invention relates to an information processing system having a communication function, and more particularly relates to an information processing system having means for controlling communication using a key input device and a machine control character.
2. Description of the Prior Art
Functions of various machines have been improved with the advance of scientific technique, and in addition, at present, the development of machines which can communicate with each other via communication paths such as network, has occurred. For example, if typewriters have a communication function apart from its essential function, it is possible to use the key board as an input device and to use the printing unit as an output device. By so doing, typewriters could have a variety of applications such as to a Key board Send and Receive terminal, or KSR terminal for an information processing system or transmission system.
The processing capabilities of the printing unit of a typewriter, however, is poor so that the operation speed is extremely lowered if the communication speed is set so as not to bring about malfunction (missing character) even during "next line", "next page" processing or the like. In this case, if the host is arranged to automatically interrupt the transmission, the communication control becomes complicated. In view of the above, by setting the communication speed at the usual printing speed of a printing unit, in the event only when processings require much time such as when the printing unit is disabled to output a received data, a protocol (functional character), for example, X.multidot.ON (DC1)/X.multidot.OFF(DC2) or ENQ/ACK can be used for controlling communication. In synchronizing the processings, for example, in the X.multidot.ON/X.multidot.OFF control, as the KSR terminal (typewriter) sends out "X.multidot.OFF(DC2)", the transmission of data to the KSR terminal from the host is interrupted, and upon reception of "X.multidot.ON(DC1)", the transmission is resumed. Alternatively, in the ENQ/ACK control, with "ACK" not transmitted or "NAK" transmitted, a disabled condition of reception is indicated, or "ACK" is transmitted only when the reception is enabled. With the synchronizations as above, the degradation of the overall efficiency due to the operation requiring a long processing time as in " next line", "next page" or the like has been avoided. However, with the key board of a KSR terminal, a control code may be produced generally by depressing a character key while depressing another particular control key. For example, if the "Q" key is depressed while the "CONTROL" key on the key board is depressed, the same code (11).sub.H as "X.multidot.ON(DC1)" is transmitted to the host. Thus, distinguishing from a control code "X.multidot.ON(DC1)" key for synchronizing control with the printing unit can not be performed, resulting in malfunction in the printing unit. Therefore, during the enabled condition of the printing unit, it has been impossible to input or transmit the above various control characters on the key board.